Nickel(II) sulfate is simple, water soluble salt of sulfuric acid. It occurs in nature as minerals retgersite (NiSO4·6H2O), morenosite (NiSO4·7H2O) and some other mixed sulfates. Crystals of Nickel(II) sulfate hexahydrate have frequent use in optics. Some of their most availed optical properties is optical rotatory dispersion that ranges from 200 nm to 2500 nm in the wavelength  and circular dichroism in the visible spectral range . Their unusual bandlike absorption spectrum in the range 200–1200 nm, which contains only three trans-mission bands (peaked at 250, 490, and 880 nm) makes them one of the most effective band filters in the solar blind spectral range. At the same time, the transmission in the UV region exceeds 80% .
Nickel(II) sulfate forms four hydrates (mono-, di- ,hexa-, hepta-,). The hydrate with the biggest amount of crystal water – NiSO4·7H2O – crystallizes from the solutions at temperatures below 31,5°C [4,5]. Heptahydrate has two polymorhs – monoclinic and orthorombic. This hydrate loose crystal water when exposed to air even at room temperatures and decompose to nickel sulfate hexahydrate.
Orthorombic crystals of NiSO4·7H2O [photo: Juraj Kmotorka]
α-polymorph of NiSO4·6H2O [photo: Juraj Kmotorka]
Αt temperatures above 53,3°C a monoclinic β-polymorph of NiSO4·6H2O crystallizes from aqueous solutions. This polymorph has green colour.
β-polymorph of NiSO4·6H2O [photo: Juraj Kmotorka]
The temperatures at which these hydrates crystallize from the solution can be changed by adding H2SO4 to a solution which is often done to increase the efficiency of growth technology .
Crystallization curve of NiSO4 
Nickel(II) sulfate was made from nickel metal powder and sulfuric acid
Ni(s) + H2SO4(aq) → NiSO4(aq) + H2(g)
An excess of sulfuric acid (75mL 95%, 0,73mol) was poured into a beaker with 100mL of distilled water. Then Nickel metal powder (29,35g, 0.5mol) was carefully added. The mixture was constantly stirred and gently heated to speed up the reaction. After the reaction ended up the solution was filtered and the nickel sulfate was recrystallized 2 times and dried. Yield: 71.33 %. The product was a mixture of hepta- and hexahydrate which has after some time fully converted to hexahydrate.
Crystallization of several forms of NiSO4:
1. Orthorombic Nickel sulfate heptahydrate: Nickel sulfate hexahydrate (44,16g, 168mmol) was dissolved in distilled water (54mL). The solution was heated to 40°C and then let to freely crystallize at room temperature.
2. Monoclinic Nickel sulfate hexahydrate: Nickel sulfate hexahydrate (63.17) was dissolved in distilled water (100mL). The solution was heated up to 85°C and slowly cooled.
3. Tetragonal Nickel sulfate hexahydrate was crystalized out from 100 mL of 30 % sulfuric acid solution containing nickel(II) sulfate hexahydrate (40g, 152mmol) at 20°C by evaporation process.
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